Dimethyl hydrogen phosphite (DMHP) has been found to be a lung carcinogen in F344/N rats but not in B6C3F1 mice. It has been reported that DMHP is unstable in aqueous solution while it is stable as solution in methanol. No further information is available on the degree of unstability of this chemical in water or in any other organic solvent. This study was initiated in order to study the stability of this chemical in water, phosphate buffer pH 7.4 and some organic solvents. The purposes of these studies are a) to select an appropriate solvent to be used as a vehicle for metabolism studies; b) to get some understanding about the chemical stability of DMHP inside the animal; and c) to plan in vitro metabolism study using subcellular preparations from rats and mice.